Dispersed polymers, including both non-aqueous dispersion (NAD) polymers and microgels, have been found useful as rheology control agents and as toughening agents in coating compositions. Such dispersed polymers are referred to as sterically stabilized microparticles. They comprise a microparticle or core component and, attached to this core component, a polymeric stabilizer component which sterically stabilizes the core component in the dispersing medium. The microparticle or core component is a macromolecular polymer which is not soluble in the dispersing medium, either because of incompatibility or, as in microgels, by substantial crosslinking. The sterically stabilized microparticles are conventionally prepared by free radical addition polymerization of monoethylenically unsaturated monomers, which constitute the core, in the presence of the stabilizing polymer component and a hydrocarbon dispersing medium.
Typically, the stabilizer component is one of two kinds: (1) a linear polymer which may have a single or a plurality of polymerizable functional groups along the polymeric chain, or (2) a graft or block polymer composed of two segments, one of which is soluble and the other insoluble in the dispersing medium. For example, U.S. Pat. No. 4,591,533 to Antonelli et al. teaches stabilizers of the first kind, are linear acrylic polymers prepared by free radical addition polymerization of monoethylenically unsaturated monomers. The stabilizers thus formed have a plurality of further polymerizable vinyl unsaturated groups distributed randomly along the polymer chain. U.S. Pat. No. 4,746,714 to Spinelli et al. discloses other stabilizing polymers of the first kind, which are also linear acrylic polymers, but which are prepared by what is referred to as group transfer polymerization process, so that the stabilizing polymers each have a single terminal functional group for attachment to a microparticle. Finally, U.S. Pat. No. 5,010,140 to Antonelli et al. discloses still other stabilizing polymers of the first kind, but which are prepared using a catalytic chain transfer agent containing cobalt. The latter stabilizing polymers also have terminal vinyl unsaturation.
Examples of the second kind of stabilizing polymers employed to sterically stabilize and disperse microparticles are disclosed in numerous patents, including U.S. Pat. No. 4,147,688 to Makhlouf et al. and U.S. Pat. No. 4,220,679 to Backhouse. Such stabilizing polymers comprise an insoluble or anchor segment, typically an acrylic polymer, to which is grafted or attached a soluble segment. Typically, the dispersing medium is a hydrocarbon solvent so that the soluble second, sediment is a molecule or polymer of low polarity. Some common examples of such soluble segments include poly(12-hydroxystearic acid), poly(2-ethylhexylmethacrylate), poly(laurylmethacrylate), and poly(stearylmethacrylate).
U.S. Pat. No. 5,025,060 and European published application EP 422647 disclose dispersed particles having a combination of two different macromonomer stabilizers A and B. However, neither macromonomer has only one terminal point of attachment to the microparticle or core, but rather have a distribution of polymerizable unsaturated double bonds for attachment.
The kind or composition of polymer employed to stabilize a micro,article determines the dispersability and compatibility of the resulting non-aqueous dispersion (NAD) polymer or microgel. Stabilizers of the first kind, indicated above, are generally good for providing non-aqueous dispersion (NAD) polymers that are compatible and well dispersed in as non-flocculated state in predominantly acrylic coating compositions. However, this may result in the use of a large amount of such dispersion polymers to achieve the desired theology control. On the other hand, stabilizers of the second kind mentioned above are generally good for providing non-aqueous dispersions or microgels that are in a relatively flocculated state in acrylic coating compositions. It is known in the art that such flocculated dispersions can provide for good shear thinning characteristics which lead to good sag resistance and/or metallic flake control in coatings. The extent of flocculation and hence shear thinning depends on the degree of incompatibility of the stabilizer with the resins comprising the coating binder. Therefore, a high degree of shear thinning is possible at low levels of stabilized microparticles through the use of stabilizers of the second kind, wherein the soluble segment of the stabilizer is highly incompatible with the coating binder resins. However, the attainment of good sag resistance or metallic flake control is generally at the expense of leveling, and a finish with unacceptable gloss and distinctiveness of image (DOI) may result. The balancing of rheology control and levelling is generally difficult. Also, the incompatibility of the stabilizer with the binder resins can result in "kick-out" of the stabilized microparticles and the formation of seed. This is especially evident in highly polar binder systems.
It would thus be highly desirable to obtain an improved stabilized microparticle that can be tailored to achieve the desired rheology control without sacrificing gloss or distinctness of image and which does not tend to kick-out when employed in the coating composition.